Standardizing electrical components in power tool applications, such as hand-held power tools, are hampered by the very different demands of the wide range of applications, particularly, the types of motors and switches used. As power tools have evolved, performance, cost and ergonomics have caused power tool manufacturers to use many different electrical configurations.
This push toward customized solutions has resulted in situations where even for a single type of application, electrical drills for example, different switch suppliers have developed different switch platforms. While these different switch platforms typically have comparable performance ratings, they tend to differ widely in the number, type, location and orientation of their terminations. Where the power tool manufacturer “dual sources” the switch, this has the undesirable effect of propagating multiple different “wire-ups” depending on the switch selected. A “wire-up” is a term commonly used to refer to the wiring arrangement used in the tool. These variations in wire-ups then necessitate different cord sets, motor lead wire terminations and lengths, as well as requiring that various peripherals such as EMI filters and electronics be connected differently.
The overall impact of having different switch platforms from different suppliers for the same power tool application results in reduced design flexibility, complicates the supply chain, and increases the potential for confusion and error during assembly of the power tool. Since the potential for customizing existing switches is limited, each new power tool that uses that switch platform tends to evolve towards a sub-optimal wire-up with more unnecessary connections as well as more complex wire routings.
Power tools, and in particular, hand-held power tools, use three main switch types depending on the application. These are push-button, in-line and overhang. Push-button switches are simply on/off switches and their main application is in small angle grinders. In-line switches are typically used in drills, hammer drills, and screw guns. In-line switches often include a variable speed control where a device such as a potentiometer controls the output of a power electronics circuit that powers a motor. They may also have an integrated mechanism to reverse the motor. Such in-line switches are often known as “variable speed reversing” or “VSR” switches. Overhang switches are used in most saw applications (e.g., miter saws, circular saws and reciprocating saws. With the exception of the overhang switches used in certain reciprocating saws, such as those having variable speed, overhang switches are also generally simply on/off switches. Overhang switches used in reciprocating saws having variable speed typically include control electronics that provides the variable speed function.
Push Button Switch
One of the challenges posed by today's push button switches is that they have a boxlike form that must be accommodated in small, handheld tools such as grinders where ergonomics are important selling features. The packaging of various electrical components in such a tool can be difficult, particularly with the advent of tools having more features which often have separate electronic controls.
With reference to FIG. 1, a typical prior art push button switch 100 has a body 102 with an internal push-button actuator and an external actuator, such as a button 104, extending from a top 106. As oriented herein, the term “top” or “front” is used to refer to the side of the switch having the actuator, such as button 104, the term “bottom” or “back” is used to refer to the side of the switch opposite the side having the actuator, and the term “side” is used to refer to the remaining sides of the switch. Push button switch 100 also has tab or screw terminals (not shown) on a bottom 108 to secure the leads, typically two, from a cord set (not shown). It also typically has tab terminals 110 (only one of which is shown) on bottom 108 to secure motor leads 112 (only one of which is shown in FIG. 1) and to secure the EMI capacitor (not shown). It should be understood that the external actuator could be other than a button, such as a rocker, a slide, a paddle, or the like. Push button switch 100 might then be known as a slide switch, rocker switch, or paddle switch, respectively.
A disadvantage of tab terminals is that when the requisite connector 114, such as a Faston type connector available from Tyco, is plugged onto the tab terminal 110, the body of the connector 114 extends well beyond the bottom 108 of the body 102 of push button switch 100. This significantly increases the axial length of the envelope occupied by the push button switch 100 and connectors that plug onto the tab terminals. This often results in the need to bend the connectors and/or severely kink the lead wires. This makes assembly difficult and can present the possibility of subsequent failure due to damaged wires or terminals.
Overhang Switch
Most overhang switch applications are relatively simple and require only on/off operation. But newer power tool applications, such as features that are becoming standard in saws, require a more complex overhang switch application. These features include a dynamic brake, such as a brake winding that is shorted through the armature of the motor when the trigger switch of the power tool is released, or an electronic brake that operates in conjunction with the run winding of the motor. Also, laser sight lines in miter and some circular saws are becoming increasingly popular and these require separate power supplies that must be wired into the overhang switch.
There are three main switch terminations typically used in overhang switches. They are tab terminals, side-mounted screw terminals (as oriented when the power tool is upright), and bottom-mounted screw terminals (again as oriented when the power tool is upright).
FIG. 2 shows an overhang switch 200 having the tab terminal type of connections. Overhang switch 200 typically has four tab terminals 202 (only two of which are shown) that extend from a body 204, illustratively with two tab terminals 202 extending from one side of switch body 204 and the other two tab terminals extending from an opposite side of switch body 204. Overhang switch 200 has a switch actuator 206, such as a trigger, at a top or front 208. (In FIG. 2, overhang switch 200 is oriented so that its bottom side is up.) Ends of leads 210 have insulated Faston type connectors 212 attached thereto and the Faston connectors are placed on tab terminals 202 to connect leads 210 to overhang switch 200. While this simplifies assembly as the Faston type connector can be placed on the tab terminals without the need to use a dedicated tool to do so, it is less than ideal if additional connections (such as may be required for a power supply for a laser sight line) are needed over and above the four tab terminals that are typically provided. Also, tab terminals typically can't handle as high a current as screw terminals and if the Faston connector isn't fully inserted over the tab terminal, it may increase the possibility of failure. Insulated Faston connectors are also more expensive than standard ring terminals used with screw terminals.
FIG. 3 shows an overhang switch 300 having the side-mounted screw terminal type of connections. Elements of overhang switch 300 in common with elements of overhang switch 200 of FIG. 2 will be identified with the same reference numbers and only the differences will be discussed. Overhang switch 300 is oriented in FIG. 3 with its bottom side up. Overhang switch 300 includes screw terminals 302 on opposite sides of switch body 204. Ring terminals 304 are affixed to ends of leads 210 and are fastened to screw terminals 302 by screws 306.
Using side-mounted screw terminals in lieu of tab terminals solves some of the above noted problems attributable to the use of tab terminals, but creates others. Screw terminals can handle higher current than Faston type connectors and allow for multiple connections. They also cost less than insulated Faston connectors and the screw connections tends to be more robust than the slip-on connection provided by Faston connectors. But the location of the screw terminals on the side of the switch bodies presents some difficulties. For example, as shown in the circled portion 307 of FIG. 3, the screw(s) 306 located directly under the trigger 206 are difficult to access. Also, to minimize the axial length of overhang switch 300, screw terminals 302 are typically not much thicker than tab terminals 202, which means that the threaded portions of screw terminals 302 are not much thicker than tab terminals 202. As such, the threaded portion of screw terminals 302 has few threads, perhaps one or less, so that the threaded engagement between screws 306 and screw terminals 302 is not particularly robust. This may result in stripped threads, such as during assembly or later service if screws 306 are over tightened. Further, since the ring terminals 304 are fastened to sides of switch body 204, the bodies of the ring terminals 304 extend beyond a bottom 308 of switch body 204. This means that the ring terminals 304 must be bent at an appropriate angle to avoid touching the inside of the handle of the power tool (not shown) having overhang switch 300. Practically, this requires that the handle of the power tool have more room behind the overhang switch 300, often resulting in the girth of the handle being larger. This can be detrimental since the width and girth of a power tool handle, particularly for power tools of the type that use overhang switches, are often important ergonomic criteria. Also, a dedicated tool is typically required to fasten the screws 306 into the screw terminals 302 during assembly of the power tool.
FIG. 4 shows an overhang switch 400 having the bottom-mounted screw terminals type of connections. Elements of overhang switch 400 in common with elements of overhang switches 200 of FIGS. 2 and 300 of FIG. 3 will be identified with the same reference numbers and only the differences will be discussed. Overhang switch 400 is oriented in FIG. 4 with its bottom side up. Overhang switch 400 includes screw terminals 402 mounted on bottom 308 of switch body 204. In addition to the advantages of using screw terminals as discussed above with respect to overhang switch 300 of FIG. 3, since screw terminals 402 are mounted on the bottom 308 of body 204, they can be thicker or include deep, threaded bushings, that minimize or even eliminate the possibility of stripped threads, both during assembly and in the event of later service. The bottom-mounted screw terminals 402 are also more ergonomic because they are easier to access. Also, the ring terminals 304 don't need to be bent nor do the leads 210 need to be kinked as the leads 210 can exit directly from the sides of the switch body 204. Further, the connections between ring terminals 304 and screw terminals 402 are flush with the bottom 308 of switch body 204.
In-Line (VSR)
In-line switches, particularly in-line VSR switches, tend to be the most complicated switches presently used in power tool applications. This is due to the electronic content of these switches, the multiple connections that they must accommodate and the multiple configurations commonly used.
There are two main schemes used in in-line VSR switches: the 4-wire (asymmetrical) wire-up and the 6-wire (symmetrical) wire-up. The 4-wire scheme is typically used in 120 VAC applications where there isn't an EMI requirement and the two coils of the field winding are connected in series on one side of the armature (hence asymmetric). In the 6-wire scheme, the 2 coils of the field winding are connected one on each side of the armature (hence symmetric).
The 4-wire scheme is illustrated in more detail in FIG. 5 for an in-line VSR switch 500 having a reversing box 518 with reversing box connections 503, 504 connected to an armature 516 of an electric motor 514 and reversing box connections 506, 507 connected to field windings 510 of a field 512 of electric motor 514. In-line VSR switch 500 also includes cord set connections 501 and 502 connected to cord set 518.
The 6-wire scheme is illustrated in more detail in FIG. 6. In the 6-wire scheme, the two field coils 600 of the field 602 of electric motor 604 are connected one on each side of the armature 606 of electric motor 604 to reversing box connections 618, 620 of a reversing box 622 of an in-line VSR switch 608 and to motor connections 630, 632 of in-line VSR switch 608. Armature 606 of electric motor 604 is connected to reversing box connections 624, 628. In-line VSR switch 608 also includes cord set connections 610 connected to a cord set 612. It also includes EMI connections 614 connected to an EMI capacitor 616. In the 6-wire scheme, the two coils of the field winding are connected one on each side of the armature 606 to utilize the inductance of field coils 600 to act as a filter for any electrical noise generated at the brush/commutator interface of armature 606 and mitigate the need for additional EMI components.
The next consideration is the form of the tool itself, which generally falls into two major classes: pistol grip and mid-handle. A pistol grip has the shape, as the name implies, of a pistol grip and the handle and switch are aft of the motor and most of the wiring enters from above or below the switch. In this configuration, terminals on the top or bottom of the switch are preferred while terminals on the side of the switch body are inconvenient since they are difficult to access and make wire routing difficult. In power tools having pistol grip handles, such as drills, width and girth of the handle are important ergonomic criteria so it is desirable not to have to increase either to make access to the terminals and/or wire routing easier.
In the mid-handle design, the handle and switch are located directly under the motor so lead wires exiting from the top of the switch are undesirable. This is further complicated by the range of terminals used by various switch manufacturers, ranging from tab terminals of various sizes, locations and orientations, to push-in type terminals. Push in type terminals are internal to the switch and typically consist of two parts—a spring arm and a supporting plate. The lead wire (or pin type terminal) is inserted between the plate and the spring arm and is secured by the spring force of the spring arm pressing it against the plate.